1. Field of the Invention
The present invention is related to a reflective liquid crystal projection apparatus.
2. Description of the Related Arts
FIG. 22 is one example of a configuration of reflective liquid crystal projection apparatus according to the prior art.
Generally, a reflective liquid crystal projection apparatus 1 (hereinafter referred to as reflective LCD projector 1) having a configuration shown in FIG. 22 is commonly known. As shown in FIG. 22, the reflective LCD projector 1 is composed of a light source 2 such as a lamp, a collimating lens 4 that collimates light emitted from the light source 2, a polarizing beam splitter 6, a reflective liquid crystal element 8 (hereinafter referred to as reflective LCD 8) that modulates polarized light in response to a picture signal S1 that is supplied to the reflective LCD 8, a projection lens 10 and a screen 12. The light, which is reflected by the reflective LCD 8 and passes through the polarizing beam splitter 6, is projected on the screen 12 by the projection lens 10.
In the reflective LCD projector 1, the light emitted from the light source 2 is collimated to be parallel approximately by the collimating lens 4 and converted into linear polarized light by the polarizing beam splitter 6, and then irradiated on the reflective LCD 8. In the reflective LCD 8, the light is modulated by the picture signal S1 and reflected. The reflected light is incident to the polarizing beam splitter 6 again, and a particular component of the reflected light is separated, and then the separated light is projected on the screen 12 through the projection lens 10. Consequently, a picture image is projected on the screen 12. Since the light that shuttles between the polarizing beam splitter 6 and the reflective LCD 8 passes through the same optical path, such a light source system of the reflective LCD projector 1 is termed as an ON-AXIS optical system.
In the case of such an ON-AXIS optical system, as long as the polarizing beam splitter is used, there existed a phenomenon such that a polarization condition for skew light is apt to be changed due to the characteristic of the polarizing beam splitter. Consequently, there existed a problem such that leaked light caused by the above-mentioned phenomenon makes it difficult to display black state excellently.
In order to solve the problem mentioned above, it is commonly practiced that inserting a ¼ wavelength (λ) plate (hereinafter referred to as λ/4 plate) into the light path between the polarizing beam splitter 6 and the reflective LCD 8 compensates a black level and obtains an excellent black state.
This kind of projection apparatus is apt to use the projection lens 10 having a larger NA (numerical aperture) so as to display a brighter picture image on the screen 12. Although increasing an NA reduces an F number and enables to make a projected picture image brighter, however, there existed another problem such that increasing the NA increases light loss in the polarizing beam splitter 6 and results in that the projected picture image is not so bright as expected.
Further, if the NA is increased, light that passes through the liquid crystal layer of the reflective LCD 8 obliquely is also used for projecting a picture image on the screen 12, and resulted in a problem of deteriorating black-and-white contrast ratio.
In addition thereto, it is strictly required for the optimum angle of the λ/4 plate to be adjusted precisely. Consequently, delicate angle adjustment as many as the order of 0.1 degree, for example, is necessary, and resulted in creating another problem that the angle adjustment becomes harder.